IMPREGNATION OF THE OVTM. 



67 



two nuclei before the commencement of segmentation. Biitschli was the 

 earliest to state from observations on Rhabditis dolichura that the first 



FIGS. 34, 35, AND 36. THREE SUCCESSIVE STAGES IN THE COALESCENCE OF THE MALE AND 



FEMALE PEONUCLEI IN AsTEEIAS GLACIALIS. FROM THE LIVING OVUM. (Copied fl'Oin Fol.) 



segmentation nucleus arose from the fusion of two nuclei, and this was 

 subsequently shewn with greater detail for Ascaris nir/rovcnosa, by 

 Auerbach (76). Neither of these authors gave at the first the correct inter- 

 pretation of their results. At a later 

 period Biitschli (So) arrived at the con- 

 clusion that in a large number of in- 

 stances (Lymnceus, Nephelis, Cucullamis, 

 &c.), the nucleus in question was formed 

 by the fusion of two or more nuclei, 

 and Strasburger at first made a similar 

 statement for Phalli tsia, though he has 

 since withdrawn it. Though Biitschli's 

 statements depend, as it seems, upon a 

 false interpretation of appearances, he 

 nevertheless arrived at a correct view 

 with reference to what occurs in im- 

 pregnation. Van Beneden (78) described 

 in the rabbit the formation of the original 

 segmentation nucleus from two nuclei, 

 one peripheral and the other central, and 

 deduced from his observations that the 

 peripheral nucleus was derived from the 

 spermatic element. It was reserved for Oscar Hertwig (89) to describe 

 in Echinus lividus the entrance of a spermatozoon into the egg and the 

 formation from it of the male prouucleus. 



The general fact that impregnation consists in the fusion of 

 the spermatozoon and ovum has now been established for some forms 

 in the majority of invertebrate groups (Arthropoda and Rotifera 

 excepted). Amongst Vertebrata also it has been shewn by E. van 

 Beneden that the first segmentation nucleus is formed by the 

 coalescence of the male and female pronucleus. Calberla, and Kupffer 

 and Benecke have demonstrated that a single spermatozoon enters at 

 first the ovum of Petromyzon. 



The contact of the spermatozoon with the egg-membrane causes in Petro- 

 myzon active movements of the protoplasm of the ovum, and a retreat 

 of the protoplasm from the membrane. 



In Amphibia the appearance of a peculiar pigmented streak 

 extending inwards from the surface of the pigmented pole of the 



FIG. 37. OVUM OF ASTERIAS GLA- 

 CIALIS, AFTER THE COALESCENCE OF THE 

 MALE AND FEMALE PRONUCLEI. (Copied 



from Fol. ) 



52 



